The present invention relates to an auxiliary power unit and more particularly to an inlet door therefore.
Aircraft auxiliary power units (APU) are gas turbine engines often located in an aircraft empennage such as an aircraft tail, fuselage wing root or fuselage wheel well area. The APU receives inlet airflow through a forward inlet silencer duct and discharges an exhaust though an acoustic muffler that is mounted to an exhaust duct of the APU to reduce noise associated therewith. The inlet typically includes a hinged door movable between three positions: a closed position, an in-flight position and a ground position.
During aircraft flight, the door extends into the free stream airflow to define a forward facing air inlet scoop. Minimizing the extent to which the door projects into the free stream airflow improves the aerodynamics of the aircraft, reduces the amount of noise that is transmitted to the cabin of the aircraft, and reduces concerns associated with the formation of ice on the door, the inlet silencer duct and plenum.
When the aircraft is on the ground, the inlet door is opened to a fully open position to compensate for the lack of free stream airflow and minimize pressure drop as a significant pressure drop may effect efficient operation of the APU. Such a fully open position, however, directs a fully open forward facing air inlet scoop toward a forward section of the aircraft such that noise may propagate therefrom. The inlet noise typically travels from the compressor through the inlet duct and is deflected from the inlet door toward the forward section of the aircraft. Since the forward section of the aircraft often include passenger doors, personnel cabins, refueling points and baggage doors, audible noise levels experienced by those onboard the aircraft or those on the ground may be increased such that the APU system will typically incorporate an inlet silencer to attenuate the APU noise and compressor tone level. Although effective, such an inlet silencer may increases weight, cost and complexity of the APU system.